Hydrocarbon feedstocks such as petroleum crudes, gas oil, etc. are subjected to various processes in order to isolate and separate different fractions of the feedstock. In refinery processes, the feedstock is distilled so as to provide light hydrocarbons, gasoline, naphtha, kerosene, gas oil, etc.
The lower boiling fractions are recovered as an over head fraction from the distillation column. The intermediate components are recovered as side cuts from the distillation column. The fractions are cooled, condensed, and sent to collecting equipment. No matter what type of petroleum feedstock is used as the charge, the distillation equipment is subjected to the corrosive activity of acids such as H.sub.2 S, HCl, organic acids, and H.sub.2 CO.sub.3.
Corrosive attack on the metals normally used in the low temperature sections of a refinery process system, (i.e. where water is present below its dew point) is an electrochemical reaction generally in the form of acid attack on active metals in accordance with the following equations:
(1) at the anode EQU Fe(s) .fwdarw.Fe.sup.++ +2e.sup.- PA1 (2) at the cathode EQU 2H.sup.+ +2e.sup.- 2H EQU 2H.fwdarw.H.sub.2 (g)
The aqueous phase may be water entrained in the hydrocarbons being processed and/or water added to the process for such purposes as steam stripping. Acidity of the condensed water is due to dissolved acids in the condensate, principally HCl, organic acids, H.sub.2 S, and H.sub.2 CO.sub.3. HCl, the most trouble some corrosive material, is formed by hydrolysis of calcium and magnesium chlorides originally present in the brines.
Corrosion may occur on the metal surfaces of fractionating towers such as crude towers, trays within the towers, heat exchangers, etc. The most troublesome locations for corrosion are tower top trays, overhead lines, condensers, and top pump around exchangers. It is usually within these areas that water condensate is formed or carried along with the process stream. The top temperature of the fractionating column is usually, but not always, maintained at about or above the dew point of water. The aqueous condensate formed contains a significant concentration of the acidic components above-mentioned. These high concentrations of acidic Components render the pH of the condensate highly acidic and, of course, dangerously corrosive. Accordingly, neutralizing treatments have been used to render the pH of the condensate more alkaline to thereby minimize acid-based corrosive attack at those regions of the apparatus with which this condensate is in contact.
One of the chief points of difficulty with respect to corrosion occurs above and in the temperature range of the initial condensation of water. The term "initial condensate" as it is used herein signifies a phase formed when the temperature of the surrounding environment reaches the dew point of water. At this point a mixture of liquid water, hydrocarbon, and vapor may be present. Such initial condensate may occur within the distilling unit itself or in subsequent condensors. The top temperature of the fractionating column is normally maintained above the dew point of water. The initial aqueous condensate formed contains a high percentage of HCl. Due to the high concentration of acids dissolved in the water, the pH of the first condensate is quite low. For this reason, the water is highly corrosive. It is important, therefore, that the first condensate be rendered less corrosive.
In the past, highly basic ammonia has been added at various points in the distillation circuit in an attempt to control the corrosiveness of condensed acidic materials. Ammonia, however, has not proven to be effective with respect to eliminating corrosion occurring at the initial condensate. It is believed that ammonia has been ineffective for this purpose because it does not condense completely enough to neutralize the acidic components of the first condensate.
At the present time, amines such as morpholine and methoxypropylamine (U.S. Pat. No. 4,062,746) are used successfully to control or inhibit corrosion that ordinarily occurs at the point of initial condensation within or after the distillation unit. The addition of these amines to the petroleum fractionating system substantially raises the pH of the initial condensate rendering the material noncorrosive or substantially less corrosive than was previously possible. The inhibitor can be added to the system either in pure form or as an aqueous solution. A sufficient amount of inhibitor is added to raise the pH of the liquid at the point of initial condensation to above 4.5 and, preferably, to between 5.5 and 6.0.
Commercially, morpholine and methoxypropylamine have proven to be successful in treating many crude distillation units. In addition, other highly basic (pKa&gt;8 ) amines have been used, including ethylenediamine and monoethanolamine. Another commercial product that has been used in these applications is hexamethylenediamine.
A specific problem has developed in connection with the use of these highly basic amines for treating the initial condensate. This problem relates to the hydrochloride salts of these amines which tend to form deposits in distillation columns, column pumparounds, overhead lines, and in overhead heat exchangers. These deposits manifest themselves after the particular amine has been used for a period of time, sometimes in as little as one or two days. These deposits can cause both fouling and corrosion problems and are most problematic in units that do not use a water wash.